In many industrial filtration processes, a filter cake is formed upon a filter cloth by supplying a suspension or slurry to one side of the cloth and applying a negative pressure at the other side of the cloth to remove liquid from the suspension or slurry. Such process is known as "dewatering", although other liquid constituents and dissolved substances may also be removed. Also, the term "filter cloth" as used herein is intended to cover any suitable or commercially available web material for filtering purposes, not just textile fabric.
Many types of apparatus are known for accomplishing the aforesaid filtration process, for example rotary drum filters, rotary disc filters, horizontal belt filters and many others.
Since vacuum induced dewatering cannot achieve total removal of moisture from the filter cake it is often necessary to follow the filtration step with a thermal drying step. Ideally this should be done in the same equipment to avoid the need for mans Opp O OO Oy0::B' heat sensitive such that drying temperatures may never exceed 80.degree. or 90.degree. C., not even during a break down. At such temperatures the overall drying time is too long in comparison with the filtration time, making the equipment impossibly large. In this respect typical filtration times in continuously operating equipment are of the order of two to three minutes, whereas drying times at such temperatures are often of the order of twenty to thirty minutes.
A further difficulty in drying the filtered product "in situ" is that upon drying the filter cake has a tendency to form a hard crust which inhibits the drying process and may make the product difficult to remove at the end.